Method for preparing zno nanopowder

ABSTRACT

Disclosed is a method of preparing ZnO nanopowder according to a non-equilibrium synthetic process, comprising adding an organic substance containing an amine group or a carboxyl group as a fuel material to an aqueous solution having Zn 2+  and (NO 3 ) ions to prepare a mixed solution, and heating the resulting solution with agitation. The method is advantageous in that the ZnO nanopowder has excellent valuable metal recovery and harmful organic substance decomposition efficiency, and the highly pure ZnO nanopowder with nano-sized particles is prepared in commercial quantities.

TECHNICAL FIELD

The present invention relates, in general, to a method of preparing zincoxide (ZnO) nanopowder and, more particularly, to a method of preparinghighly pure microscopic semiconductor nanopowder in commercialquantities with excellent recovery efficiency of valuable metalsexisting in industrial waste according to a new non-equilibriumsynthetic process.

BACKGROUND ART

As well known to those skilled in the art, photocatalytic reactions area field having a relatively short history in the field of catalyticchemistry. The function of a catalyst is mediated by interaction betweena surface of the catalyst and reactant molecules, and the interactionbetween the catalyst and the reactant molecule generally accompaniestransfer of electrons between the catalyst and the molecule.

Accordingly, a semiconductor being readily capable of controlling theconcentration of electrons therein is frequently used in the study ofcatalytic reactions. Initial studies on photocatalysts mostly related totechnologies of converting solar energy into other types of energy andstoring the solar energy but, recently, studies of treating waste water,waste, or purification of air using photocatalysts have attractedconsiderable attention.

Various semiconductor materials are used as photocatalysts. Thesesemiconductor materials should have high optical activity and stability,be capable of using visible light or ultraviolet light, and below-priced so as to be practically used in a photocatalytic reaction.

Materials of photocatalysts are classified into metallic complexesrepresented by chlorophyll, and semiconductors. In particular,semiconductor oxides have been actively studied as photocatalystsbecause of their broad excited energy band gap and ease of handling.Among various semiconductor oxides, ZnO, an n-type semiconductor oxidewith excess metals, which has a Wurtzite structure, acts as acrosslinking accelerator in the rubber industry, and is applied to avaristor in the electronic industry, phosphor in FED, and aphotocatalyst, is growing in importance.

Meanwhile, nanopowder has peculiar physical and chemical properties incomparison with bulky materials. The nanopowder has high activity, lowsintering temperature, and large specific surface area. In addition, thenanopowder may have high purity according to a method of preparing thenanopowder. Accordingly, it is expected that activity of the catalyst isimproved due to increase of a catalyst surface area and variation of itssurface properties such as surface defects when the nanopowder isapplied to the catalyst.

Furthermore, materials used as the desirable photocatalyst should bestable in a solution when a beam having higher energy than the band gapof the material is irradiated to the material, and readily dispersed soas to improve particle efficiency, that is to say a ratio of measuredsurface area to theoretical total surface area of particles constitutingthe material. Accordingly, it is important to prepare a stable andwell-dispersed nanopowder so as to improve optical activity of thephotocatalyst.

Conventional methods of preparing zinc oxide are classified into a vapormethod and a sol-gel method. However, the vapor method isdisadvantageous in that it is actually impossible to prepare nano-sizedZnO because zinc oxide is formed in the shape of agglomerates due todifficulty in controlling reaction conditions. On the other hand, thesol-gel method has disadvantages in that stringent control of reactionconditions is needed even though uniform zinc oxide powder may be formedbecause of a violent hydrolysis reaction under atmosphere, and it isvery costly to prepare zinc oxide in commercial quantities because ofexpensive alkoxide used as a reactant. Therefore, this method isattempted on laboratory level.

DISCLOSURE OF THE INVENTION

Accordingly, the present invention has been made keeping in mind theabove problems occurring in the prior art, and an object of the presentinvention is to provide a novel method of preparing zinc oxide powder asphotocatalytic semiconductor powder with nano-sized particles incommercial quantities.

In order to accomplish the above object, the present invention providesa method of preparing ZnO nanopowder, comprising adding an organicsubstance containing an amine group or a carboxyl group as a fuelmaterial to a starting material solution having Zn²⁺, (NO₃)⁻, and (OH)⁻ions to prepare a mixed solution, and heating the mixed solution withagitation.

Moreover, the present invention further provides a method of preparingZnO nanopowder characterized in that the organic substance containingthe amine group or the carboxyl group is selected from the groupconsisting of carbohydrazide, oxalic dihydrazide,1-methyl-3-nitroguanidine, ammonium perchlorate, urea hydrogen peroxide,and guanidine nitrate in the above method.

Further, the present invention further provides a method of preparingZnO nanopowder characterized in that the mixed solution is prepared bydissolving Zn(NO₃)₂·6H₂O and the fuel material in distilled water in abeaker in the above method.

Furthermore, the present invention further provides a method ofpreparing ZnO nanopowder characterized in that the mixed solution isprepared by dissolving Zn(OH)₂ and nitric acid in distilled water andthen adding the fuel material in the above method.

In addition, the present invention further provides a method ofpreparing ZnO nanopowder characterized in that the starting materialsolution is mixed with the fuel material in a non-equilibrium state suchthat an oxidation number ratio of the starting material solution to thefuel material is not 1 in the above method.

The present invention also provides a product for removing harmful gas,treating industrial waste, or purifying air, including ZnO nanopowderprepared by such method.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a X-ray diffraction pattern of zinc oxide powder according tothe present invention;

FIG. 2 is a transmission electron microscope picture illustrating shapeand size of particles constituting zinc oxide powder according to thepresent invention;

FIG. 3 is a graph comparing efficiencies of photocatalysts with eachother when recovering Ag from each powder sample;

FIG. 4 is a graph comparing efficiencies of photocatalysts with eachother when recovering Cu from each powder sample; and

FIG. 5 is a graph showing the result of a decomposition test of organicsubstance in waste water using various photocatalysts.

BEST MODE FOR CARRYING OUT THE INVENTION

A detailed description of the present invention will be given, below.

According to the present invention, a method of preparing zinc oxide(ZnO) nanopowder is performed in accordance with following procedure. Aninitial solution containing metal ions (oxidant) is prepared, and a fuelmaterial is then added to this solution. The initial solution containsions such as Zn²⁺, (NO₃)⁻, and (OH)⁻ and for example, may be prepared bydissolving Zn(NO₃)₂·6H₂O or Zn(OH)₂ powder in nitric acid. Then the fuelmaterial is added, whereby the fuel material is selected from the groupconsisting of glycine (H₂NCH₂COOH), carbohydrazide (H₂NNHCONHNH₂),oxalic dihydrazide, 1-methyl-3-nitroguanidine, ammonium perchlorate,urea hydrogen peroxide, and guanidine nitrate.

Meanwhile, the present invention is based on a non-equilibrium syntheticprocess modified from a general glycine-nitrate process (GNP). The GNPis conducted under an equilibrium state due to spontaneous combustion ofthe fuel, that is to say an oxidation number ratio of the oxidant to thefuel =1, adjusted by controlling each oxidation number of an oxidant(starting material) and the fuel. For example, when glycine is used asthe fuel, the oxidation number of the oxidant is calculated andcontrolled to prepare ZnO powder containing unreacted commutedimpurities according to the GNP. On the other hand, in the presentinvention, after each oxidation number of the oxidant and fuel iscalculated, an aqueous solution is prepared in a non-equilibrium state,that is to say, excess oxidant or fuel is added to the solution tospontaneously combust the fuel, unlike the glycine-nitrate process. Atthis time, the oxidation number ratio of the oxidant to the fuel isnot 1. This process according to the present invention is defined as anon-equilibrium synthetic process.

An amount of each agent added to the solution is quantitativelycalculated according to the chemical reaction equations below, andagents are added to the solution in the amount within a desirable range(in the non-equilibrium state) based on a calculated amount of theagent. Chemical reactions occurring in the present invention may berepresented as follows:Zn(OH)₂+2HNO₃→Zn(NO₃)₂+2H₂O (aqueous solution state, starting material:Zn(OH)₂)Zn(NO₃)₂+2H₂O+fuel→ZnO+xN₂↑+yCO₂↑

The resulting solution is heated by a hot plate to a temperature capableof boiling water (for example about 80 to 200° C.), with agitation usinga magnetic bar.

After distilled water is vaporized, the solution is converted intoviscous liquid phase to form small bubbles and emit gas. The resultingliquid is then put in a collection device to react nitrate groups withthe fuel to instantaneously generate very high heat (about 1500 to 1700°C.) and high pressure to cause explosive combustion, thereby preparingmetal oxide, that is to say, zinc oxide (ZnO) powder.

A better understanding of the present invention may be obtained throughthe following example which is set forth to illustrate, but is not to beconstrued as the limit of the present invention.

EXAMPLE

1. Preparation of Mixed Solution Samples

1) 0.05 mole Zn(NO₃)₂·6H₂O and 0.044 mole glycine were put into a beakerand dissolved using 300 ml of distilled water in the beaker to prepare amixed solution sample 1.

2) 0.05 mole Zn(NO₃)₂·6H₂O and 0.0666 mole carbohydrazide were put intoa beaker and dissolved using 300 ml of distilled water in the beaker toprepare a mixed solution sample 2.

3) 0.05 mole Zn(OH)₂ powder was dissolved in 300 ml of distilled watercontaining 8.25 g of 13.4 M nitric acid solution, and 0.44 mole glycinewas then added and dissolved in the resulting solution to prepare amixed solution sample 3.

4) 0.05 mole Zn(OH)₂ powder was dissolved in 300 ml of water containing8.25 g of 13.4 M nitric acid solution, and 0.0666 mole carbohydrazidewas then added and dissolved in the resulting solution to prepare amixed solution sample 4.

2. Preparation and Analysis of Zinc Oxide Nanopowder Having High Purity

Four mixed solution samples thus prepared were heated by a hot platewith agitation using a magnetic bar, respectively. After distilled waterwas vaporized, the solutions were converted into viscous liquids to formsmall bubbles and emit gas.

Each resulting liquid was then put in a collection device to beexplosively combusted with generation of high heat, thereby producingthe metal oxide white zinc oxide (ZnO) powder in the shape of sphere orrod. At this time, size and shape of the particles constituting thepowder depended on the starting material and fuel.

In the case of powder including sphere-shaped particles, Zn(OH)₂ wasused as the starting material and glycine was used as the fuel, and inthe case of powder having rod and plate-shaped particles, Zn(NO)₃ wasused as the starting material and carbohydrazide was used as the fuel.

The ZnO nanopowder was subjected to a heat treatment at 400° C. so as toremove a small amount of NO₃ gas remaining on the surface of the ZnOnanopowder, thereby obtaining finally the target ZnO powder.

ZnO nanopowder prepared from the mixed solution sample 3 wasqualitatively analyzed according to an X-ray diffraction method toconfirm crystallinity of the powder, and an X-ray diffraction pattern ofthe powder is shown in FIG. 1. FIG. 2 is a transmission electronmicroscope picture illustrating shape and size of particles constitutingzinc oxide powder according to the present invention, in which size ofeach particle was extremely minute in the range of tens of nanometers.

3. Photocatalyst Effect

(1) Silver (Ag) Recovery Test

CeO₂ synthesized under the same conditions as the present invention,TiO₂ synthesized according to a conventional HPPLT (homogeneousprecititation process at low temperature) process, TiO₂ manufactured byDegussa Co. of Germany, and ZnO powder prepared from the mixed solutionsample 3 of the present invention each were dipped in waste watercontaining silver and irradiated by ultraviolet light to test therecovery performance of silver by photocatalytic effect.

As shown in FIG. 3, it took 45 minutes to completely recover silver fromthe waste water (i.e. until a silver concentration in the waste water is0) in the case of using TiO₂ manufactured by Degussa Co., known as themetal oxide with the best performance among conventional metal oxides.On the other hand, by using ZnO powder of the present invention, it tookonly 15 minute to completely recover silver from the waste water.Accordingly, the powder of the present invention has three times betterperformance than the conventional best powder.

(2) Copper (Cu) Recovery Test

Powder used in the above test was tested in the view of recoveryperformance of copper ions, and the results as shown in FIG. 4. In FIG.4, HPPLT (slurry) was a slurry type of TiO₂ synthesized according to aconventional HPPLT process, and nanotube was a nanotube type of TiO₂.From the test results, it could be seen that a Cu ion concentration wasnot reduced to 3% or less even though ultraviolet light was irradiatedto waste water for a long time in the case of TiO₂ powder manufacturedby Degussa Co., but the Cu ion concentration was 0 after an irradiationtime of about 5 minutes, thereby completely recovering Cu ions in thecase of using ZnO powder of the present invention.

(3) Organic Substance Decomposition Test

The same samples as those used in the above test were tested fordecomposition performance of organic substances in waste water. FIG. 5is a graph illustrating a total organic carbon (TOC) concentration inthe waste water as a function of irradiation time of ultraviolet rays.These results showed that a concentration of organic substance was 50%or higher after the irradiation time of 300 minutes in the case of usingTiO₂ powder manufactured by Degussa Co., but ZnO powder of the presentinvention recovered most organic substances in about 15 is minutes,thereby proving excellent organic substance decomposition ability of ZnOpowder according to the present invention.

Industrial Applicability

As described above, a method of preparing ZnO powder according to thepresent invention is advantageous in that highly pure ZnO nanopowderhaving superior valuable metal recovery and organic substancedecomposition efficiency, compared to a conventional photocatalyticpowder, is prepared in commercial quantities.

Although the preferred embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

1. A method of preparing ZnO nanopowder, comprising: adding an organicsubstance containing an amine group or a carboxyl group as a fuelmaterial to a starting material solution having Zn²⁺, (NO₃)⁻ and (OH)⁻ions to prepare a mixed solution; and heating the mixed solution withagitation.
 2. The method according to claim 1, wherein the organicsubstance containing the amine group or the carboxyl group is selectedfrom the group consisting of glycine, carbohydrazide, oxalicdihydrazide, 1-methyl-3-nitroguanidine, ammonium perchlorate, ureahydrogen peroxide, and guanidine nitrate.
 3. The method according toclaim 1, wherein the mixed solution is prepared by dissolvingZn(NO₃)₂·6H₂O and the fuel material in distilled water in a beaker. 4.The method according to claim 1, wherein the mixed solution is preparedby dissolving Zn(OH)₂ and nitric acid in distilled water and then addingthe fuel material.
 5. The method according to claim 1, wherein thestarting material solution is mixed with the fuel material in anon-equilibrium state such that an oxidation number ratio of thestarting material solution to the fuel material is not
 1. 6. A productfor removing harmful gas, treating industrial waste, or purifying air,comprising ZnO nanopowder prepared by the method according to claim 1.7. A product for removing harmful gas, treating industrial waste, orpurifying air, comprising ZnO nanopowder prepared by the methodaccording to claim
 2. 8. A product for removing harmful gas, treatingindustrial waste, or purifying air, comprising ZnO nanopowder preparedby the method according to claim
 3. 9. A product for removing harmfulgas, treating industrial waste, or purifying air, comprising ZnOnanopowder prepared by the method according to claim
 4. 10. A productfor removing harmful gas, treating industrial waste, or purifying air,comprising ZnO nanopowder prepared by the method according to claim 5.